Researchers applied heat to the skin of 17 people with arthritis and nine people without the disease, and found that the more opiate receptors in the brain, the higher a person's ability to withstand pain. Opiate receptors are proteins in the brain that link up with narcotic painkillers and help reduce feelings of pain.

PET scans also showed the arthritis patients had more opiate receptors, which seems to be an adaptive response to help them cope with their chronic pain, said Christopher Brown and colleagues at the University of Manchester in England.

"As far as we are aware, this is the first time that these changes have been associated with increased resilience to pain and shown to be adaptive," Brown said in a university news release.

The study doesn't prove, however, that arthritis pain actually led to the increase in opiate receptors.

Still, Brown said, "although the mechanisms of these adaptive changes are unknown, if we can understand how we can enhance them, we may find ways of naturally increasing resilience to pain without the side effects associated with many pain-killing drugs."

The study was published Oct. 23 in the journal Pain.

"This is very exciting because it changes the way we think about chronic pain," Anthony Jones, director of the Manchester Pain Consortium, said in the news release.

There is generally a negative and fatalistic view of chronic pain -- pain that extends beyond six months, he said. "This study shows that although the group as a whole are more physiologically vulnerable, the whole pain system is very flexible and that individuals can adaptively upregulate their resilience to pain," he said.

"It may be that some simple interventions can further enhance this natural process, and designing smart molecules or simple non-drug interventions to do a similar thing is potentially attractive," Jones said.